A dump truck mounts a vessel pivotally up and down pivoted to a pivot portion provided on a rear part of a vehicle body and providing, as means for rotating the vessel up and down, hoist cylinders, which are composed of hydraulic cylinders, pivoted between the vehicle body and the vessel. Using, as a hydraulic pressure source, a hydraulic pump driven by a prime mover, such as an engine, mounted on the dump truck, and switching an oil passage between the hydraulic pump and the hoist cylinders by a main control valve, the hoist cylinders are allowed to extend or retract.
As regards operation modes of the vessel, there are a RAISE operation, a HOLD operation, a FLOAT operation, and a LOWER operation. The RAISE operation is an operation that by supplying hydraulic oil to bottom chambers of the hoist cylinders, the hoist cylinders are allowed to extend to raise the vessel. The HOLD operation is an operation that the vessel is held in a current posture by cutting off hydraulic circuits, which lead to the hydraulic pump and an oil tank, relative to the rod chambers and bottom chambers of the hoist cylinders. The FLOAT operation means an operation that allows the vessel to descend by its own weight by having the rod chambers and bottom chambers of the hoist cylinders short-circuited through the main control valve. The LOWER operation means an operation that forcedly lowers the vessel by supplying hydraulic oil to the rod chambers of the hoist cylinders. This LOWER operation is performed when the vessel does not descend by its weight on such an uphill or the like that the dump truck becomes high at its front part.
As conventional hoist systems for performing such four operations, there are one described in Patent Document 1 and another one described in Patent Document 2. The hoist system described in Patent Document 1 includes one directional control valve as a main control valve. This directional control valve is composed of a 4-position selector valve, with a RAISE position where a RAISE operation is performed, a HOLD position where a HOLD operation is performed, a FLOAT position where a FLOAT operation is performed, and a LOWER position where a LOWER operation is performed, being arranged in this order. Of these four positions, the HOLD position is a neutral position, the RAISE position is a switched position on a side opposite to the FLOAT position and LOWER position, and the FLOAT position is located between the HOLD position and the LOWER position. This 4-position selector valve is switched in the same direction for the FLOAT position and LOWER position, so that a common pilot pressure receiving part is used for the FLOAT position and LOWER position.
The hoist system described in Patent Document has a hydraulic circuit configured as shown in FIG. 5. In FIG. 5, there are shown an engine 1 mounted on a vehicle body of a dump truck, hydraulic pumps 2, 3 driven by the engine 1, and hoist cylinders for rotating a vessel up and down. These hoist cylinders 4 are provided as many as two in total, one each on the left and right of the vehicle body of the dump truck. Also shown are 3-position selector valves 5, 6 as main control valves for controlling the hoist cylinders 4. By changing the combination of operation and non-operation of these 3-position selector valves 5, 6 as will be described subsequently herein, they perform RAISE operation, HOLD operation, FLOAT operation and LOWER operation.
There are also shown a RAISE pilot valve 7, a FLOAT pilot valve 8, and a LOWER pilot valve 9. These pilot valves are switched by control signals from a controller (not shown) to supply pilot pressure oil to the 3-position selector valves 5, 6. Also shown are a reducing valve 24 for reducing the pressure of oil, which has been delivered from the hydraulic pump 2, to provide pilot pressure oil, relief valves 10, 11 for preventing overloading of the hoist cylinders 4, and check valves 12, 13 connected in series to the relief valves 10, 11, respectively. There is further shown a variable relief valve 14 for setting a maximum pressure for oil to be delivered from the hydraulic pump 3. This variable relief valve 14 has a pilot pressure receiving part 14a that raises the relief pressure upon a RAISE operation.
In this conventional circuit, the 3-position selector or valves 5, 6 remain in the neutral positions, respectively, while none of the pilot valves 7, 8, 9 is operated. In these neutral positions, circuits 15, 16, which are connected to bottom chambers 4a and rod chambers 4b of the respective hoist cylinders 4, are cut off by the 3-position selector valves 6, 5 so that the vessel is held in the same position.
When the RAISE pilot valve 7 is switched to the low position by the controller from the state shown in the drawing, the 3-position selector valves 5, 6 are both switched to the left positions and at the same time, pilot pressure oil is supplied to the pilot pressure receiving part 14a of the variable relief valve 14, whereby the set pressure of the variable relief valve 14 rises. Then, the oil delivered from the hydraulic pump 3 is supplied to secondary circuits 17, 18 of the 3-position selector valves 5, 6, merges back together at a junction 19, and is supplied to the bottom chambers 4a of the hoist cylinders 4 through the circuit 15. On the other hand, the oil in the rod chambers 4b of the hoist cylinders 4 is returned to an oil tank 21 through the circuit 16, a circuit 20 and the 3-position selector valve 5.
When the FLOAT pilot valve 8 is operated and switched to the low position, on the other hand, only the 3-position selector valve 5 is switched to the right position. The bottom chambers 4a of the hoist cylinders 4 whereby come into communication with the oil tank 21 through the circuits 15, 17 and 3-position selector valve 5. Here, it is to be noted that the rod chambers 4b have already been connected to the oil tank 21 through the circuits 16, 20, the check valve 13 and a circuit 25. It is also to be noted that the rod chambers 4b have already been connected to the oil tank 21 through the circuits 16, 22, check valve 26 and circuit 25. When the hydraulic pressure in the bottom chambers 4a rises by the own weight of the vessel, a portion of the hydraulic oil in the bottom chamber 4a, therefore, circulates to the rod chambers 4b through the circuits 15, 17, 3-position selector valve 5, circuit 25, check valve 13 and circuits 20, 16. At this time, another portion of the working oil, which has flowed out of the 3-position selector valve 5, circulates through the circuit 25, check valve 26 and circuit 22, merges at a junction 23 with oil from the circuit 20, and circulates into the rod chambers 4b. Extra working oil, which occurs by a difference in cross-sectional area between the bottom chambers 4a and the rod chambers 4b, is returned to the oil tank 21. Accordingly, the vessel is allowed to descend.
Further, when the LOWER pilot valve 9 is operated and switched to the low position, only the 3-position selector valve 6 is switched to the right position. The hydraulic oil from the hydraulic pump 3 is whereby supplied to the rod chambers 4b of the hoist cylinders 4 through the 3-position selector valve 6 and circuits 22, 16, and the hydraulic oil in the bottom chambers 4a is drained to the oil tank 21 through the circuits 15, 18 and 3-position selector valve 6. Therefore, the hoist cylinders 4 retract so that the vessel can be forcedly lowered. During this LOWER operation, no pilot pressure is supplied to the variable relief valve 14, so that a hydraulic pressure lower than that at the time of a RAISE operation is applied to the rod chambers 4b. 